The above-mentioned specific group of hypochloritestable naphthotriazolyl stilbene fluorescent whitening agents are described in U.S. Pat. No. 2,784,103, issued to Keller, Zweidler and Hausermann, on March 5, 1957. They were described as having an affinity to textile fibers, such as cellulose, polyamide, polyurethane, polyester and blends thereof. As this affinity occurs from aqueous solutions, they lend themselves to inclusion in powdered and liquid detergent compositions.
The preferred compound of this class is the sodium salt of 2-(stilbyl-4")-(naphtho-1',2':4,5)-1,2,3 triazole-2"-sulfonic acid.
The compound, marketed under the trademark Tinopal.RTM. in large quantities, is manufactured by the following scheme: ##STR1##
The present invention is directed to step C, the oxidation step wherein the monoazo compound is converted to the final product by ring-closure of the monoazo intermediate to the triazole final product.
Until this invention, the ring-closure step has been performed by air oxidation of the solution of the monoazo sodium salt in aqueous 2-butoxyethanol (Butyl Cellosolve), (BC), (Dowanol EB). The oxidation was performed by sparging air at atmospheric pressure through the aqueous solution of the monoazo salt in 2-butoxyethanol at the boiling point of the azeotrope of the solvent mixture (about 90.degree.-95.degree. C.). The reaction solution consisted of a 5-6 wt. % solution of the monoazo in an alkaline aqueous solution of 35 wt. % of 2-butoxy ethanol. It was carried out at the reflux of the azeotrope. A copper sulfate catalyst promoted the oxidation so that it could be completed within about 4-7 hours after initiating the air-sparging. After the oxidation was completed, as indicated by spot tests, the air-sparging was halted and Na.sub.2 S.sub.2 O.sub.4, sodium hydrosulfate (sodium dithionite), was added to decompose trace amounts of colored impurities.
The physical properties of the reaction solvent, 2-butoxyethanol, hereinafter BC, and its mixtures with water have a great influence on the oxidative ring-closure of the monoazo to the triazole. Mixtures of this ether with water in the range between 5 and 75 wt. % of BC form a constant boiling azeotrope (b.p. 98.8.degree. C.). This limits the maximum temperature available in the atmospheric oxidation process to 98.8.degree. C.
During oxidation the triazole formed must remain in solution. If the triazole precipitates during the oxidation, solvent-containing red-colored by-products become trapped in the triazole crystals and the final product will be unacceptable. To prevent precipitation of the triazole, a minimum amount of BC (approximately 33 wt. %) has been required.